It is known from various studies that the relative degree of conversion of alkali cellulose with etherification agents increases as the molar ratio of sodium hydroxide solution to cellulose (.mu..sub.o) is reduced. The molar amount of etherification agent which reacts with the alkali cellulose in the desired manner, based on the total molar amount of etherification agent, is hereafter referred to as the selectivity. The selectivity decreases with the increase in the molar ratio of sodium hydroxide to cellulose, from approximately 85 percent at a .mu..sub.o of 1.5 to approximately 35 to 45 percent at .mu..sub.o -values of 4, according to measurements made. The figures mentioned for the selectivity apply to the use of ethylene oxide as well as to the use of methyl chloride. Other known etherification agents produce substantially the same results. The strong decrease in selectivity can be explained by the increased amount of water that is added during the alkalization reaction with 50 percent hydroxide solution. Undesirable secondary reactions are thereby increased.
In the technically wide-spread dipping alkalization of cellulose sheets and boards the relationship between selectivity and .mu..sub.o is taken into account by the degree of pressing out and by the dipping in of sodium hydroxide solutions of different concentrations (from 20 to 45 percent). In the preparation of relatively highly substituted cellulose ethers, the alkalization must be carried out with from 35 to 45 percent sodium hydroxide solution.
The preparation of alkali cellulose from fibrous crumblike as well as from powdered cellulose may take place in double shaft extruders [German Published Applications (DE-AS) Nos. 15 43 114 and 15 43 136] with from 16 to 48 percent sodium hydroxide solution, or in a closed compulsory mixer equipped with a fast mixing mechanism, by spraying a 50 percent sodium hydroxide solution [German Published Application (DE-AS) No. 26 35 403]. These methods also result in a disproportionately great loss of etherification agent due to secondary reactions during the preparation of highly substituted mixed cellulose ethers. The preparation of a mixture of powdered cellulose and caustic soda in a mill is also known [German Published Application (DE-OS) No. 14 68 226].